Wastewater treatment plants are continually researching ways to make discharged, treated sewage water safer for the environment. Over the past few decades there has been an increased concern with respect to the levels of biological nutrients, primarily phosphorous, in the treated wastewater that is discharged into the environment. As a result, many governmental bodies have introduced stricter regulations for the permissible phosphorous level in treated wastewater. In some cases these regulations have presented a challenge to treatment plant managers. As the trend toward stricter regulations continues, this challenge is likely to become increasingly widespread.
Phosphorous is introduced into the water system mainly via human waste, synthetic detergents and fertilizers. High levels of biological phosphorous in discharged wastewater can lead to eutrophication in local waters, which may cause changes in fauna speciation and fish kills.
The treatment of sewage is largely a biochemical operation where living microorganisms, principally bacteria, carry out chemical transformations of the sewage. Different environments favor the growth of varying populations of microorganisms and this in turn affects the efficiency, end products, and completeness of wastewater treatment.
In many waste water treatment facilities nutrients, including phosphorous, are removed by way of the action of a specific type of bacteria, which requires volatile fatty acids (VFAs) for use as an energy source.
Traditionally, primary sludge from the first stage of wastewater treatment (the primary clarifier) has been fermented to create VFAs for later use in biological nutrient removal processes like the Enhanced Biological Phosphorous Removal (EBPR) or the de-nitrification process. The fundamental biological processes of fermentation are the same as those of the first stage of anaerobic digestion, which is composed of two phases, hydrolysis and acetogenesis. The products of hydrolysis are soluble organic acids, which contain carbon. Acetogenesis converts those acids to volatile fatty acids (VFAs). VFAs are converted to methane by methanogenic bacteria in the second stage of anaerobic digestion. If diverted to a biological nutrient removal process the VFAs provide the energy source for the Enhanced Biological Phosphorous Removal process and a carbon source for de-nitrification.
With currently available technology, it is difficult to produce a liquid stream that has a wide range of VFA concentrations to meet the varying demands of the biological nutrient removal processes, and sludge that has a high percent total solids concentration and with good mechanical properties that permits easy pumping. There are several variations of fermentation methods that are currently used. The most basic is to allow primary sludge to stratify in a tank. VFAs are produced in the liquid and sludge fractions, but only the VFAs in the liquid portion are available to be pumped to the biological nutrient removal process. The high concentrations of VFAs in the sludge are sent to the solids handling processes and are not available for nutrient removal. If the amount of VFAs needed is reduced by seasonal or wastewater changes and the Sludge Residence Time (SRT) of the fermenter is reduced to reduce VFA production the sludge will not have enough SRT to thicken. The reduction in thickening will produce a less concentrated sludge with a lower percent total solids. A lower percent total solids sludge will increase solids handling and disposal costs. In an attempt to extract and recover the high concentrations of VFAs in the sludge, the xe2x80x98activatedxe2x80x99 primary process subjects the sludge to anaerobic conditions to begin the production of organic acids via fermentation. A portion of this xe2x80x98activatedxe2x80x99 sludge is then recycled back to the input line where it is mixed with incoming primary sludge. During this mixing process the primary sludge is inoculated with actively fermenting organisms for further organic acid production and elutriation of VFAs. One variation of fermentation, referred to as the xe2x80x9ccomplete mixxe2x80x9d method, is to force mixing in the fermentation tank before the sludge is separated by gravity. Extra stages allowing for further fermentation before recycling are sometimes added to this process.
One disadvantage of recycling activated anaerobic sludge is that over time biological growth may cause the sludge to become so viscous that it cannot be easily transported through the plant""s pipelines. The sludge also never becomes well settled and therefore does not allow for optimal disposal, increasing costs to the wastewater plant. Another disadvantage of activated fermented sludge recycling is that as the sludge recycles over and over it gets older and bacteria that consume VFAs to produce methane increase in population.
Current treatment methods may not be able to satisfy the increasingly strict environmental regulations governing the allowed amount of phosphorus in treated wastewater. Satisfying these regulations becomes particularly difficult when the wastewater itself does not contain sufficient easily degradable organic matter to generate VFAs in a quantity large enough to achieve the newly desired level of biological nutrient removal. Some conditions that may allow this problem to occur are: a xe2x80x98weakxe2x80x99 influent, such as may occur during heavy rainfalls; low operating temperatures, such as during winter months or in colder geographical zones; or, short residence times in the pipes leading to the treatment plant.
The amount of VFAs needed for biological phosphorous removal production varies throughout the year depending on individual plant conditions. Excess VFAs can cause operational problems such as filament growth, bulking sludge and an increase in aeration requirements. A deficiency in VFAs also causes problems for the plant.
For some plants, in order to comply with increasingly stringent regulations, efficiency of the removal of phosphorous must be increased. When VFA production is insufficient, the plant must enhance the biological removal of phosphorous. This can be done by the addition of supplemental carbon sources such as acetic acid. Phosphorous removal can also be done chemically with the use of metal salts such as aluminum salts or iron salts. Metal salts primarily functions as a coagulant that aids in the removal of particulate material. Metal salts also promote the removal of phosphorous by facilitating chemical precipitation, which settles with the sludge and is removed along with the sludge.
This alternative may not only be expensive, but it also produces a chemical sludge that tends to be xe2x80x98fluffy,xe2x80x99 thereby increasing overall sludge volume. A higher volume makes it harder to remove water from the sludge, which also makes the sludge harder to dispose of, thereby increasing disposal costs.
Other drawbacks of this alternative include the need for extra storage space for the chemicals. Chemicals used in this process, such as Alum, tend to be highly corrosive. Having such chemicals on site at the wastewater treatment facility increases safety concerns and liability for the plant.
What is needed, but is not yet available, is a method of fermentation that consistently produces a desired concentration of volatile fatty acids to meet the varying demands of biological nutrient removal and efficient thickening of sludge while allowing for economical construction and operating costs of the wastewater treatment plant.
The present invention is a method for treating wastewater that generates a supernatant having a concentration of volatile fatty acids (VFAs) that falls within a specified range and a sludge that has specified percent total solids independent of VFA production. The method includes the steps of providing a fermentation tank and a gravity thickener, operatively connected to the fermentation tank. The primary sludge is permitted to flow continually or intermittently into the fermentation tank and the liquid portion of the wastewater overflows the fermentation tank, thereby defining an overflow rate. The fermentation tank is subjected to anaerobic conditions and the wastewater in the tank is permitted to stratify so that sludge forms at the bottom of the tank. In addition the sludge is continually pumped from the bottom of the fermentation tank so that each stratification level within the fermentation tank corresponds to a sludge residence time that in turn corresponds to a range of VFA concentrations. The overflow from the fermentation tank is combined with the sludge from the fermentation tank in second tank acting as a gravity thickener, permitting VFAs to be separated from the sludge and mixed into the overflow to form a supernatant rich in VFAs. The sludge is conditioned by elutriation to improve its settling characteristics. The rate of the pumping of the sludge from the bottom of the thickener tank being set such that the sludge has the specified percent total solids desired. VFA production may be increased or decreased by varying the SRT in the fermenter while thickening can be controlled independently in the gravity thickener to optimize sludge thickening independently of VFA production.
The foregoing and other objectives, features and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.